Session Initiation Protocol (SIP) is a widely adopted standard used for, among other things, Voice over IP (VoIP). SIP is used for session initiation, modification, and termination. Despite numerous revisions, SIP follows a common trend in VoIP protocol architecture and is designed primarily as a first party protocol, where the party executing the SIP commands is usually one of the parties involved in the communication.
The first-party design of SIP and other VoIP protocols makes certain advanced features difficult to implement. For example, Push to Talk also known as ‘walkie talkie’ features are difficult to implement in the current SIP standard and existing hardware. Other features such as an ‘Information Tone’, ‘Click to Call’, and ‘Remote Monitoring’ are similarly difficult to implement. In some cases, such features may be possible if extensions to the SIP standard were implemented.